Petra might be situational, but with the right situation it will be the most powerful wonder. Imo The wonder I never get is Pentagon.
I find Petra to be worth it even if you only have 1 desert tile. The free caravan plus an extra trade route is a big bonus. If you actually have desert tiles around to improve it's absolutely incredible. If you have more than 6 desert tiles it actually yields more extra food than HG as well as hammers. Petra is insanity.